Analysis of steady state chloride transport through two heterogeneous field soils

Chloride transport was investigated in a loamy soil and a silty-loam soil at the field scale under steady state flow conditions using a water flux of 2.84 cm d^-1 for the loamy and 1.5 cm d^-1 for the silty-loam soil. The solute plume movement was recorded by means of horizontally installed time domain reflectrometry (TDR) probes at 5 depths up to 90 cm below the soil surface and 24 locations along a transect of 8 m. The measurements consisted of solute resident concentrations every 2 hours in the loamy soil for a total period of 42 days and every 4 hours for the silty-loam soil for a period of 65 days. Parameters of the convective-dispersive equation (CDE) and the lognormal stochastic-convective transport model (CLT) were determined using time-normalized resident concentration breakthrough curves C(rt*) (z, t). In addition, temporal moments of C(rt*) (z,t) were related to travel time moments and transport parameters for the two transport processes. At both sites the breakthrough curves at different depths were better described by the CLT than by the CDE. However, early solute breakthrough was underestimated at most depths. Mean travel time and dispersivity were estimated using the temporal moments of C(rt*) (z, t) with the assumption of a stochastic-convective transport process. In the loamy soil, solute was traveling from a heterogeneous, macroporous top soil toward a subsoil containing significantly fewer macropores. The flow of solutes through the macropores is not detected by the TDR probes, resulting in a larger observed mean travel time compared with the expected mean travel time based on the piston flow model and no increase in dispersivity. In contrast, in the subsoil the observed and expected mean travel times were in good agreement, and dispersivity increased with depth. In the silty-loam soil, mean travel times derived from concentration measurements were larger than the expected mean travel times based on the piston flow model, implying temporal storage of solutes in stagnant water zones. Dispersivity also showed deviations from the expected linear increase with depth, probably because of changing soil properties with depth.